The present invention relates to the field of communication, and more particularly, to the impedance matching and adjustment of a multiple-resonance frequency of a circularly-polarized plane antenna used for satellite communication. Further, the present invention relates to a portable radio employing a circularly-polarized plane antenna.
The concept of a portable cellular phone using satellites has recently been proposed by various corporations. With regard to frequency bands used for the portable cellular phone, a frequency band of 1.6 GHz is assigned to up-link communications from a ground portable cellular phone to a communications satellite, and a frequency band of 2.4 GHz is assigned to down-link communications from the communications satellite to the ground portable cellular phone. The frequency band of 1.6 GHz is also assigned to bi-directional communications between ground stations and the communications satellite. A circularly-polarized wave is commonly used in the communications in order to ensure the quality of a communications circuit.
A plane antenna has already been in actual use which receives a radio wave (e.g., a circularly-polarized right-turn wave of 1.5 GHz) transmitted from a Global Positioning System (GPS) satellite. The plane antenna is a one-point back feeding microstrip antenna (MSA) comprising a plate-like dielectric substance, a patch conductor (i.e., a radiation element) labeled to one side of the plate-like dielectric substance, and a ground conductor labeled to the other side of the plate-like dielectric substance. FIG. 5 is a view showing an existing one-point back feeding microstrip antenna (MSA) 21 when viewed from directly above, and a patch-shaped conductor 21b has a rectangular parallelepiped shape. Taking the length of longer sides PO and QR of a patch conductor 21b as L and the length of shorter sides PQ and OR of the patch conductor 21b as S, the conductor is set such that 100.times.L/S=102 to 103% or thereabouts. The longer sides PO and QR produce resonance at comparatively low frequencies and demonstrate an elliptically-polarized wave. In contrast, the shorter sides PQ and OR produce resonance at comparatively higher frequencies and demonstrate another elliptically-polarized wave orthogonal to the previously-described elliptically-polarized wave. The patch conductor acts as a circular polarization antenna between the foregoing frequencies.
To connect an electric feed line having a characteristic impedance of 50 .OMEGA. a feed pin 21a (from behind), the impedance of the electric feed line is matched to that of the feed pin by adjusting the position of the feed pin 21a. More specifically, it is known that all you have to do is to place the feed pin 21a in any position along substantially-diagonal lines of a square.
A dielectric substrate 21c forming the MSA 21 has already been in actual use in the form of a dielectric substrate having a dielectric constant of about 20, a thickness of 4 to 6 mm, and a size of about 25 mm. A GPS requires a very narrow bandwidth of the order of about 1 MHz.
In contrast, since a satellite portable cellular phone performs transmission and receipt of a signal in a comparatively broader bandwidth of the order of about 10 MHz, the thickness of the dielectric substrate 21c must be increased to thereby comparatively broaden the bandwidth. Further, in a system employing a low orbiting satellite, there is a need to ensure the gain of an antenna at a low elevation angle.
However, in a case where the dielectric substrate is increased (so as to become about twice as thick as an existing GPS MSA) with a view to improving the characteristics of the antenna in a bandwidth or at a low elevation angle, it is difficult for a rectangular patch conductor to simultaneously satisfy a desired multiple resonance frequency and impedance matching.